Blowout preventers (BOP) are used on some wellheads as a method of sealing a wellbore in the event of an emergency situation in oil and gas wells. Within the wellbore, and running through the blowout preventer there is usually some form of a tubular, generally drill pipe, or some form of tool string.
Conventional ram-type blowout preventers consist of a pair of opposed rams located either side of the wellbore in a blowout preventer housing. In the event of an emergency situation, the rams are driven towards each other and either seal against the tubular in the well bore to seal the annulus between the tubular and the wellbore, or sever the tubular or tool string in the wellbore to substantially seal the well. The driving mechanism for the rams is generally hydraulically driven, and a significant hydraulic force is required to propel the rams, particularly if severance is the objective.
Conventional blowout preventers require different rams to perform the sealing operation to those required for the severing operation. Furthermore, in some instances, a pair of conventional blowout preventer rams cannot seal against different diameters of tubular running through the preventer housing. If different diameters of tubular are to be sealed against, rams are provided for each diameter of a tubular.
A further disadvantage of conventional blowout preventers is the size of the housing required to store the rams in normal, non-emergency use. In an emergency situation, the rams move from a storage position, perpendicular to the axis of the wellbore into the wellbore, and, accordingly, a significant amount of space is required either side of the wellbore to accommodate both the rams and the associated drive mechanism.